1. Field of the Invention
This invention relates to a method of selectively extracting low molecular weight solvents to concentrate higher molecular weight solutes in solution therein by the use of cross-linked ionic gels. Cross-linked ionic gels have been developed as size selective extraction solvents. Such gels absorb low molecular weight solvents, such as water, but not high molecular weight solutes, such as proteins. The high molecular weight solute is recovered as a concentrated solution. The gels are easily regenerated for reuse be a change in pH, composition and/or temperature of the surrounding liquid.
Separation processes are a key aspect of the chemical industry. In the past, these processes were dominated by distillation, reflecting the key role played by petroleum. Other important separations include gas scrubbing, liquid-liquid extraction, crystallization and filtration. However, there are an emerging group of separation problems for which current technology is expensive and energy intensive. These problems center around dilute solutions or organic or biological materials. Examples include the removal of water from dilute solutions like cheese whey, the concentration of antibiotics in fermentation beers, and the recovery of protein products of genetically-engineered microorganisms.
2. Prior Art
The basic idea of using gels as size-selective extraction solvents which can be regenerated by phase transitions seem to be new and no relevant prior art is known. However, the use of gels for separations is not new, and the study of phase transitions in gels is well established.
Separations using gels are usually based on gel permeation chromatography (GPC). The basic apparatus used in this method consists of a packed bed of gel spheres of the same size. The spheres are swollen with solvent to a constant extent. Solvent flows steadily through this bed. At time zero, a pulse of solution containing several high molecular weight solutes is injected at the top of the bed. As the pulse is swept down the column, different solutes are retained by the gel to different degrees. Basically, small solutes which can diffuse quickly into the gel are retarded the most, and large solutes which are excluded from the gel are swept along fastest. Thus the largest solutes are eluted most quickly, and the smallest solutes come out of the column last. The differential retention by any single gel sphere is very slight, but the total retention for all the spheres in the bed can effectively separate the solutes.
In the GPC separation, the separation is of very small amounts of similar high molecular weight solutes in a packed bed of gel swollen to a constant extent. Changes in swelling ruin the separation. In this invention, the separation is of potentially large amounts of a high molecular weight solute and a small solvent using a gel whose swelling is deliberately altered. Changes in swelling are central to regeneration and reuse.
Some separations using commercial gels are based on gel absorption, with resulting volume changes. These commercial gels absorb organic molecules. They are hard to regenerate.
The separation closest to producing the results of the present invention is ultrafiltration. For that process, water and small molecular weight solutes are forced under high pressure through a size selective membrane, while larger molecular weight solutes are retained as concentrates. The membranes can be used for many separations and can be cleaned by reversing the flow of solvent. However, initial membrane cost is high and the application of pressure is energy costly.